Aerosol Synthesis of Layered Oxide Cathodes Materials for Lithium-ion Batteries

Abstract

With the global population increasing and energy demand on the rise, the reliance on fossil fuels for energy generation has exacerbated climate change through the release of greenhouse gases. Rechargeable batteries, particularly lithium ion batteries (LIBs), play a crucial role in efficiently storing renewable energy, making them pivotal in the global transition towards sustainable energy. The LIB market is experiencing significant growth, primarily driven by the shift to electric vehicles. The importance of cathodes in shaping battery performance becomes evident when considering the often higher specific capacity exhibited by anode materials. Enhancing cathode materials and improving their electrochemical performance can be achieved through the refinement of synthesis methods and conditions, including the selection of metal and lithium sources, synthesis atmosphere, and annealing parameters such as temperature, duration, and atmosphere. Various synthesis techniques have been employed for LIB cathode materials, including precipitation, solid-state, sol-gel, hydrothermal, spray pyrolysis (SP), and combustion methods. Among these, coprecipitation involves multiple steps and increases processing costs, while SP offers continuous processing, scalability, and rapid homogenous composition attainment. However, SP requires additional heat treatment to achieve high purity and crystallinity, necessitating extensive experimentation to determine optimal annealing conditions. In this study, two distinct SP reactor configurations were developed: the initial design and the optimized design. These designs incorporated enhancements compared to conventional SP synthesis methods, with improvements made to the droplet preheating zone and the collection system, aiming to increase production rates at the laboratory scale. These reactors were employed for the synthesis and optimization of different cathode materials, including LiCoO2 (LCO) and LiNi0.8Mn0.1Co0.1O2, known as NMC811, which is challenging to synthesize. To gain a deeper understanding of LCO synthesis through SP, the study investigated the influence of reactor wall temperature on the structural and morphological properties of the synthesized particles using precursors of nitrate and acetate. In-situ heated X-ray diffraction (HT-XRD) was utilized to monitor the structural evolution of the particles at various temperatures and identify the optimal annealing conditions. The phase evolution was observed, with spinel Li2Co2O4 forming at lower annealing temperatures and layered oxide LCO emerging at intermediate temperatures. However, higher temperatures led to structural defects due to lithium loss. The annealed particles with the desired structure were subsequently tested in batteries, contributing to a better understanding of LCO synthesis via SP and its potential applications in electrochemical systems. Furthermore, the study investigated the impact of synthesis temperature and precursor type in the SP synthesis of NMC811 particles. HT-XRD was employed to study the structural evolution of the synthesized particles and determine the optimum annealing conditions for two different precursors and heating atmospheres. The particles exhibited a transformation from a rock-salt structure to a spinel phase, followed by a transition into a well-ordered layered structure upon lithiation and ordering. However, at higher synthesis temperatures, structural degradation occurred due to oxygen vacancies and lithium loss, as well as Li and Ni ions mixing. This process involved the migration of Li+ ions to the transition metal layer (TM), while Ni2+ ions from the TM layer moved to the Li layer. The electrochemical performance of the particles annealed under optimal conditions was thoroughly examined. Additionally, the study explored the enhancement of NMC811 cathode material synthesis using the optimized SP reactor and an electrochemical testing protocol. Factors such as reactor temperature, precursor concentration, reacting gas orientation in the preheating zone, organic additives (urea), annealing temperature, time, and high oxygen flow rate were investigated. The optimized reactor design, featuring improved reacting gas flow, resulted in enhanced particle sphericity and excellent electrochemical performance. Despite the morphological enhancements, annealing led to the formation of porous agglomerates, often demonstrating improved electrochemical performance due to enhanced lithium diffusion These findings offer valuable insights into the aerosol synthesis of Li-ion battery cathode materials, paving the way for further optimization and advancement in energy storage technologies.